[seankmartin]

This would allow future work to change from one full fragment per ID to many without
needing to change the complete file layout. Additional summary of what tried to do with core API with aim to have skeleton/api.ts pretty much final for this PR:

1. Enable the manifest concept for future proofing. This also introduces fragments, and edges between fragments.
   Catmaid still has one fragment and one fetch.
2. Make clear what spatial and not, to allow for precomputed and other by ID only sources in future.
3. Normalise names in our API with existing neuroglancer names (chunks, not cells), signal direct param not inside
   options and also with existing concepts.
4. Remove catmaid specific things that leaked into the generic API.
5. Remove limited scope bounds, use coordinateSpace to describe the full model space instead.
6. Remove lack of flexibility in node (vertex) attributes. Use an enum to describe the specially handled node
   attributes, and leave the rest as normal. Allows hooking these into the shader at a later date but that part not
   done yet as focus is on the API structure. Description remains separate as it is a string.
7. Try write a few clariyfing comments in the API about the contract and expectations
8. Remove levels allowed to be in any order, contract now expects them sorted. Catmaid now sorts within the
   datasource part itself.
9. Make SkeletonNode fully readonly, matching rest of API.

As a way to stress test this API, it adds support for precomputed skeletons in the skeleton browser